Method of fabricating a copper damascene structure

ABSTRACT

The present invention provides a chemical-mechanical polishing slurry for use in removing copper overlaying a tantalum-based barrier layer during the fabrication of a copper damascene structure, and a method of retarding the corrosion of copper lines during the chemical-mechanical polishing of a copper damascene structure using the slurry. The slurry according to the invention includes an oxidizing agent that releases free radicals and a non-chelating free radical quencher that is effective to retard the corrosion of the copper lines during chemical-mechanical polishing. Preferred oxidizing agents that release free radicals used in the slurry according to the invention include peroxides, peroxydiphosphates, and persulfates. Preferred non-chelating free radical quenchers used in the slurry according to the invention include ascorbic acid, thiamine, 2-propanol, and alkyl glycols, with ascorbic acid being most preferred.

FIELD OF INVENTION

[0001] The present invention relates to a slurry for chemical-mechanicalpolishing copper damascene structures, and more particularly, to aslurry that retards copper line corrosion during the chemical-mechanicalpolishing of copper overlaying a tantalum-based barrier layer during thefabrication of copper damascene structures.

BACKGROUND OF THE INVENTION

[0002] Integrated circuits are made up of millions of active devicesformed in or on a substrate such as silicon or gallium arsenide. Theactive devices are typically isolated from each other usingsilicon-based dielectric materials. The active devices are usuallyformed in multiple layers that are interconnected to form functionalcircuits and components. Interconnection of active devices is typicallyaccomplished through the use of well-known multilevel interconnectionprocesses such as the process disclosed in Chow et al., U.S. Pat. No.4,789,648.

[0003] Copper is a highly preferred electrically conductive material foruse in fabricating integrated circuits because it has superiorelectromigration resistance and lower resistivity than many otherelectrically conductive materials such as aluminum. Copper wiring andinterconnects allow for the use of higher critical current in integratedcircuits, which can greatly improve the performance capabilities of suchdevices.

[0004] The use of copper in integrated circuits, however, does presentsome difficult challenges. Copper readily diffuses into conventionalsilicon-based dielectric materials such as polysilicon,single-crystalline silicon, silicon dioxide, low-k inorganic and organicmaterials, and the like. Once these silicon-based materials have beencontaminated with copper atoms, the dielectric constant of thesilicon-based dielectrics is adversely affected. In addition, oncesemiconductive silicon-based materials are copper doped, transistorsmade within or in close proximity to the copper doped silicon-basedregions either cease to function properly or are significantly degradedin electrical performance. Therefore, a barrier layer or liner film mustbe applied to the silicon-based dielectric layer in order to preventcopper diffusion.

[0005] One of the presently preferred methods of fabricating integratedcircuits having copper wiring and interconnects, which are also known ascopper damascene structures, generally comprises providinginterconnected copper wiring or metallization patterns in discretelayers of dielectric film. The materials typically used to form thesedielectric film layers include phosphosilicate glass,borophosphosilicate glass, and silicon dioxide. The dielectric layer isetched or otherwise processed to pattern a series of trenches and/orholes therein. A thin barrier layer or liner film, generally not morethan approximately 300 Å thick is then deposited over the patterneddielectric layer. Preferred barrier layers or liner films comprise thinfilms of tantalum (Ta) or tantalum nitride (TaN) or both Ta and TaNdisposed over one another to form a Ta/TaN stack. Such liners arecommonly deposited by physical vapor deposition, which is also known assputter deposition, or they may be deposited by a chemical vapordeposition to form a more conformed coating. The Ta and/or TaN linercoats the surfaces of the trenches and holes as well as the uppersurface of the dielectric layer to prevent copper atom diffusion andalso to provide good adhesion between the copper layer and thedielectric layer. A layer of copper approximately 3,000-15,000 Å thickis then deposited over the liner layer so as to completely fill thetrenches and/or holes. The filled trenches thus form a network of copperlines whereas the filled holes form vias or interconnects. The finalstep in the process of fabricating an integrated circuit, which is alsoknown as the copper damascene process, is removing the copper layer andtantalum-based barrier layer from the upper surface of the dielectricfilm layer leaving only the copper filled trenches and holes. This istypically accomplished by chemical-mechanical polishing.

[0006] In a typical chemical-mechanical polishing process, themetallized surface of the copper damascene structure is placed in directcontact with a rotating polishing pad at a controlled downward pressure.A chemically reactive solution commonly referred to as a “slurry” ispresent between the pad and the surface of the copper damascenestructure during polishing. The slurry initiates the polishing processby chemically reacting with the surface of the metal film beingpolished. The polishing process is facilitated by the rotationalmovement of the pad relative to the substrate and the presence of theslurry at the film/pad interface. Polishing is continued in this manneruntil the desired film or films are removed.

[0007] The composition of the slurry is an important factor indetermining the rate at which metal film layers are removed bychemical-mechanical polishing. If the chemical agents in the slurry areselected properly, the slurry can be tailored to provide effectivepolishing of specific film layers at desired polishing rates whileminimizing the formation or creation of surface imperfections ordefects. In some circumstances, the polishing slurry can preferablyprovide controlled polishing selectivities for one or more thin filmmaterials relative to other thin-film materials.

[0008] Prior art chemical-mechanical polishing slurries used to removecopper overlaying tantalum-based barrier layers have exhibited a highselectivity for copper as compared to the tantalum-based materials. Thisadvantageously permits the rapid removal of the copper layer overlayingthe tantalum-based barrier layer. However, the aggressive chemicalaction of these prior art polishing slurries disadvantageously tends tocorrode the copper lines of the copper damascene structure duringpolishing, resulting in failure of the active devices or inconsistencyin their performance.

[0009] A need exists for an improved chemical-mechanical polishingslurry for use in removing copper overlaying a tantalum-based barrierlayer during the fabrication of a copper damascene structure. Such animproved chemical-mechanical polishing slurry would preferably removecopper overlaying tantalum-based barrier layers at a high enough rate toinsure acceptable throughput while at the same time retarding thecorrosion of copper lines.

SUMMARY OF INVENTION

[0010] The present invention provides a slurry for chemical-mechanicalpolishing copper damascene structures, and more particularly, to aslurry that retards copper line corrosion during the chemical-mechanicalpolishing of copper overlaying a tantalum-based barrier layer during thefabrication of copper damascene structures. The slurry according to theinvention comprises an oxidizing agent that releases free radicals and anon-chelating free radical quencher that is effective to retard thecorrosion of said copper lines during chemical-mechanical polishing. Theoxidizing agents that release free radicals used in the slurry accordingto the invention are preferably selected from the group consisting ofperoxides, peroxydiphosphates, and persulfates. The non-chelating freeradical quenchers used in the slurry according to the invention arepreferably selected from the group consisting of ascorbic acid,thiamine, 2-propanol, and alkyl glycols, with ascorbic acid being mostpreferred. The non-chelating free radical quenchers in thechemical-mechanical polishing slurry according to the inventionsurprisingly retard copper line corrosion during the polishing of copperdamascene structures without reducing the copper polishing rate tounacceptable levels. The anti-corrosion effect is independent of pH, butwhen the pH of the slurry is adjusted to from about 4.0 to about 7.0,the removal rate of copper is maximized.

[0011] The foregoing and other features of the invention are hereinaftermore fully described and particularly pointed out in the claims, thefollowing description setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however, of but afew of the various ways in which the principles of the present inventionmay be employed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0012] In co-pending application Ser. No. 09/277,454, the specificationof which is hereby incorporated by reference, it was disclosed that thegeneration of relatively large concentrations of free radicals cansubstantially increase the rate of copper removal duringchemical-mechanical polishing. An undesirable side-effect of thepresence of relatively large concentrations of free radicals duringchemical-mechanical polishing is copper line corrosion.

[0013] The term “corrosion” as used in the specification and in theappended claims is intended to refer to the phenomenon wherein usuallyirregular-shaped pits or depressions are chemically etched into thecopper lying in the trench or hole during the chemical-mechanicalpolishing of copper damascene structures. As used in the specificationand in the appended claims, the term “corrosion” is not intended torefer to the usually shallow depressions in the copper lying in thetrench or hole that can occur due to the chemical-mechanical action ofthe polishing pad, which is commonly known as “dishing.”

[0014] It is well-known in the art that benzotriazole (BTA) andcompounds having similar functionality can be added tochemical-mechanical polishing slurries to protect copper lines fromcorrosion. As explained in Sasaki et al., U.S. Pat. No. 5,770,095,copper reacts with BTA to form a secure film comprising a copper chelatecompound or complex. The copper chelate film serves as a protectivebarrier film to prevent oxidization or corrosion of the underlyingcopper film by the chemical agents in the slurry. Unfortunately, when asmall amount of BTA is used, it is very difficult to control theconsistency of the slurry performance. And, when a larger amount of BTAis used, the copper removal rate is decreased to an unacceptably lowlevel.

[0015] The present invention provides a chemical-mechanical polishingslurry that can protect copper lines from corrosion without the need forBTA or other chelates. Applicants have surprisingly found that copperline corrosion can be effectively suppressed by incorporating anon-chelating free radical quencher in a chemical-mechanical polishingslurry without reducing the copper polishing rate to unacceptablelevels. Accordingly, the slurry according to the invention comprises anoxidizing agent that releases free radicals and a non-chelating freeradical quencher that is effective to retard the corrosion of saidcopper lines during chemical-mechanical polishing.

[0016] The oxidizing agent used in the slurry according to the presentinvention releases free radicals during polishing. Suitable oxidizingagents for use in the invention include, for example, peroxides,peroxydiphosphates, persulfates, and combinations of the foregoing.Presently, the most preferred oxidizing agents for use in the slurryaccording to the invention are hydrogen peroxide, ammonium persulfate,and/or potassium persulfate. The oxidizing agent preferably comprisesfrom about 0.01 % to about 15.0% by weight of the slurry. Morepreferably, the oxidizing agent comprises from about 0.1% to about 10.0%by weight of the slurry. When hydrogen peroxide is used, the oxidizingagent optimally comprises from about 0.5% to about 5.0% by weight of theslurry.

[0017] The slurry according to the present invention also comprises atleast one non-chelating free radical quencher. As used in thespecification and in the appended claims, the term “non-chelating freeradical quencher” is intended to refer to a compound that does notreadily chelate or otherwise complex with copper, but that is capable ofreacting with a free radical species to retard its reactivity and thusprotect the copper metal from corrosion. The presently most preferrednon-chelating free radical quencher for use in the slurry according tothe invention is ascorbic acid, which is also known as Vitamin C. Otherpreferred non-chelating free radical quenchers include thiamine(3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-5-(2-hydroxyethyl)-4-methylthiazoliumchloride), which is also known as Vitamin B₁, 2-propanol, and alkylglycols. It will be appreciated that derivatives and precursors of thesenon-chelating free radical quenchers can also be used in the invention,and that the appended claims should be read with sufficient breadth toencompass such derivatives and precursors.

[0018] The preferred amount of non-chelating free radical quencher usedin the slurry is the smallest amount that is effective to retard thecorrosion of said copper lines during chemical-mechanical polishing.Typically, the non-chelating free radical quencher will comprise fromabout 0.01% to about 5.0% by weight of the slurry. When ascorbic acid isused as the non-chelating free radical quencher, an amount from about0.1% to about 1.0% by weight of the slurry is usually effective inretarding the corrosion of copper lines.

[0019] Optionally, the slurry according to the present invention canfurther comprise abrasive particles. However, it will be appreciatedthat for some applications it may be preferable for the slurry not tocontain abrasive particles. In such circumstances, the mechanicalpolishing action is provided by the pressure of the polishing pad. Whenpresent, abrasive particles further assist in performing the function ofmechanical grinding.

[0020] Abrasive particles that may be used in the slurry according tothe present invention may comprise any one or a mixture of a variety ofabrasive particles that are conventionally utilized inchemical-mechanical polishing slurries. Examples of suitable abrasiveparticles include alumina, ceria, copper oxide, diamond, iron oxide,nickel oxide, manganese oxide, silica, silicon carbide, silicon nitride,tin oxide, titania, titanium carbide, tungsten oxide, yttria, andzirconia, and combinations thereof. Presently, the preferred abrasivesare silica, alumina, and ceria, with alumina and silica being mostpreferred.

[0021] The abrasive particles preferably have a mean size ranging fromabout 0.02 to about 1.0 micrometers, with a maximum size of less thanabout 10 micrometers. It will be appreciated that while particle size isnot per se critical, if the abrasive particles are too small, then thepolishing rate of the slurry can be unacceptably low. It will also beappreciated that if, on the other hand, the particles are too large,then unacceptable scratching can occur on the surface of the articlebeing polished. The abrasive particles can be present in the slurry inan amount up to about 60% by weight of the slurry, more preferably fromabout 0.5% to about 30.0% by weight of the slurry, and optimally withinthe range of from about 3.0% to about 10.0% by weight of the slurry.

[0022] Preferably, the slurry according to the invention furthercomprises a solvent. The preferred solvent for use in the slurryaccording to the invention is deionized water. Other solventsconventionally used in chemical-mechanical polishing slurries can alsobe used. The slurry may also contain optional surfactants, pH adjusters,pH buffers, anti-foaming agents, and dispersing agents, which are wellknown.

[0023] The anti-corrosion effect produced by incorporating anon-chelating free radical quencher in the slurry according to theinvention is not pH dependent. In other words, the corrosion retardantphenomenon is observed throughout a broad range of pH. However, when thepH of the slurry is adjusted to from about 4.0 to about 7.0, the rate ofdesired copper removal is optimal. Preferably, the pH of the slurry isadjusted by the addition of nitric acid, potassium hydroxide, and/orammonium hydroxide.

[0024] Preferably, the slurry according to the present invention isprepared by dispersing the oxidizing agent in the solvent either beforeor after the non-chelating free radical quencher has been added. Ifabrasive particles are to be included in the slurry, the slurry can beprepared in a number of ways. For example, the slurry may be prepared bydispersing the abrasive particles in a solvent either before or afterthe oxidizing agent and/or the non-chelating free radical quencher havebeen added. The slurry may also be prepared as a two-component system(i.e., an abrasive dispersed in deionized water component and anoxidizing agent and non-chelating free radical quencher in deionizedwater component). The slurry may also be prepared in concentrated formneeding only the addition of deionized water to dilute the concentrate(or concentrated components in a two-component system) to the desiredlevel.

[0025] Alternatively, the slurry according to the present invention maybe formed by incorporating a portion of the components of the slurry ina polishing pad. For example, the abrasive particles and thenon-chelating free radical quencher could be incorporated directly inthe polishing pad, either with or without abrasive particles, anddeionized water and the oxidizing agent could then be added to the pador the surface of the article being polished to form the polishingslurry in situ. In another alternative embodiment, the abrasiveparticles could be bonded to the polishing pad, and the oxidizing agent,non-chelating free radical quencher and deionized water could be addedeither separately or together to the pad or the surface of the articlebeing polished to form the polishing slurry in situ. It will beappreciated that the components of the slurry according to the inventioncould be combined in various ways to form the slurry in situ.

[0026] It is also possible to form the components of the slurry bycombining chemical precursors together either before or at the time ofpolishing. Thus, as used in the specification and in the appendedclaims, the term “slurry” should be understood to refer to thecomponents present at the interface between the polishing pad and thesurface of the article being polished during chemical-mechanicalpolishing, and, unless otherwise stated, use of the term “slurry” isintended to encompass situations where precursors are combined to formthe components of the slurry in situ.

[0027] The present invention is also directed to a method of removingcopper overlaying a tantalum-based barrier layer during the fabricationof a copper damascene structure having a plurality of copper lines. Themethod according to the invention comprises providing achemical-mechanical polishing slurry comprising an oxidizing agent thatreleases free radicals and a non-chelating free radical quencher that iseffective to retard the corrosion of said copper lines duringchemical-mechanical polishing, and polishing the copper layer using theslurry until the tantalum-based barrier layer is exposed.

[0028] The following examples are intended only to illustrate theinvention and should not be construed as imposing limitations upon theclaims.

EXAMPLE

[0029] Slurry A was formed by dispersing 3.0% by weight aluminaparticles having an average particle diameter of 340 nm, 3.0% by weighthydrogen peroxide, and 1.0% by weight glycine in deionized water. SlurryB was formed in the same manner and had the same composition as SlurryA, except that it also contained 1.0% by weight ascorbic acid. Asufficient amount of potassium hydroxide was then added to each slurryto adjust the pH to 5.8.

[0030] Identical Sematech 926 patterned wafers (each wafer comprised asilicon substrate having an etched TEOS CVD silicon-dioxide dielectriclayer 3000 nm, a 25 nm Ta/TaN barrier layer, and 1600 nm copper layersequentially applied thereto) was separately polished with Slurries Aand B described above using a Strasbaugh 6CA polisher and a Rodel IC1400K-grooved pad. The polishing conditions were: 4 psi down pressure; 0 psiback pressure; 40 rpm table speed; 40 rpm quill speed; 20° C.temperature; and 200 cc/min slurry flow rate.

[0031] After the copper layer was removed to expose the Ta/TaN barrierlayer by chemical-mechanical polishing, the surface of each wafer wasexamined using an optical microscope for copper line corrosion. Thewafer polished using Slurry A showed severe copper line corrosionwhereas the wafer polished using Slurry B, which contained anon-chelating free radical quencher (i.e., 1.0% by weight ascorbicacid), showed no visible corrosion. The results of the foregoing Exampleare summarized in Table I below: TABLE I Agent for Cu OxidizingQuenching Other Polishing Cu Line Slurry Agent Abrasive RadicalsAdditives Rate Corrosion A 1% H₂O₂ 3% Al₂O₃ NONE 1% 900 Severe glycinenm/min B 1% H₂O₂ 3% Al₂O₃ 1% ascorbic 1% 620 NONE acid glycine nm/min

[0032] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and illustrative examplesshown and described herein. Accordingly, various modifications may bemade without departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed:
 1. A chemical-mechanical polishing slurry for use inremoving copper overlaying a tantalum-based barrier layer during thefabrication of a copper damascene structure having a plurality of copperlines, said slurry comprising: an oxidizing agent that releases freeradicals; and a non-chelating free radical quencher that is effective toretard the corrosion of said copper lines during chemical-mechanicalpolishing.
 2. The slurry according to claim 1 wherein said non-chelatingfree radical quencher is selected from the group consisting of ascorbicacid, thiamine, 2-propanol, alkyl glycols, and combinations of theforegoing.
 3. The slurry according to claim 2 wherein said non-chelatingfree radical quencher comprises from about 0.01% to about 5.0% by weightof said slurry.
 4. The slurry according to claim 1 wherein saidoxidizing agent that release free radicals is selected from the groupconsisting of peroxides, peroxydiphosphates, persulfates, andcombinations of the foregoing.
 5. The slurry according to claim 4wherein said oxidizing agent that releases free radicals is selectedfrom the group consisting of hydrogen peroxide, ammonium persulfate, andpotassium persulfate.
 6. The slurry according to claim 4 wherein saidoxidizing agent that release free radicals comprises from about 0.01% toabout 15.0% by weight of said slurry.
 7. The slurry according to claim 1further comprising abrasive particles.
 8. The slurry according to claim7 wherein said abrasive particles are selected from the group consistingof silica, alumina, ceria, and combinations of the foregoing.
 9. Theslurry according to claim 7 wherein said abrasive particles comprisefrom about 0.1% to about 60.0% by weight of said slurry.
 10. The slurryaccording to claim 1 further comprising deionized water.
 11. The slurryaccording to claim 1 further comprising pH adjusters and/or pH buffers.12. The slurry according to claim 11 wherein the pH has been adjusted toless than about 7.0.
 13. A chemical-mechanical polishing slurry for usein removing copper overlaying a tantalum-based barrier layer during thefabrication of a copper damascene structure having a plurality of copperlines, said slurry comprising: from about 3% to about 10% by weight ofabrasive particles; from about 0.5% to about 5.0% by weight of anoxidizing agent that releases free radicals; and from about 0.1% toabout 1% by weight of a non-chelating free radical quencher that retardsthe corrosion of said copper lines during chemical-mechanical polishing.14. The slurry according to claim 13 wherein said abrasive particles areselected from the group consisting of alumina, silica, and combinationsof the foregoing.
 15. The slurry according to claim 13 wherein saidoxidizing agent that releases free radicals is selected from the groupconsisting of hydrogen peroxide, ammonium persulfate, and combinationsof the foregoing.
 16. The slurry according to claim 13 wherein saidnon-chelating free radical quencher is selected from the groupconsisting of ascorbic acid, thiamine, 2-propanol, alkyl glycols, andcombinations of the foregoing.
 17. The slurry according to claim 14wherein said oxidizing agent that releases free radicals is selectedfrom the group consisting of hydrogen peroxide and ammonium persulfate,and said non-chelating free radical quencher comprises ascorbic acid.18. The slurry according to claim 17 having a pH of from about 4.0 toabout 7.0.
 19. A method of removing copper overlaying a tantalum-basedbarrier layer during the fabrication of a copper damascene structurehaving a plurality of copper lines, said method comprising: providing achemical-mechanical polishing slurry comprising: an oxidizing agent thatreleases free radicals; a non-chelating free radical quencher effectiveto retard the corrosion of said copper lines during chemical-mechanicalpolishing; and optional abrasive particles; and polishing said copperlayer using said slurry until said tantalum-based barrier layer isexposed.
 20. The method according to claim 19 wherein said optionalabrasive particles are selected from the group consisting of alumina andsilica, said oxidizing agent that releases free radicals is selectedfrom the group consisting of hydrogen peroxide and ammonium persulfate,and said non-chelating free radical quencher is selected from the groupconsisting of ascorbic acid, thiamine, 2-propanol, alkyl glycols, andcombinations of the foregoing.
 21. The method according to claim 20wherein said non-chelating free radical quencher is ascorbic acid, andwherein said slurry has a pH of from about 4.0 to about 7.0.